Lecture 24 Early Universe -Testing Models

1
Lecture 24Early Universe -Testing Models

• ASTR 340
• Fall 2006
• Dennis Papadopoulos

Chapters 12- 13
2
The Big Bang-Brief Introduction

• What were conditions like in the early universe?
• What is the history of the universe according to
  the Big Bang theory?

3
BACKGROUND THE STRUCTURE OF MATTER

• Atom is made up of
• Nucleus (very tiny but contains most off mass)
• Electrons (orbit around the nucleus)
• Atom held together by attraction between
  positively-charged nucleus and negatively-charged
  electrons.
• Binding Energy eV 3000-8000 K
• Electron mass .5 MeV 5x109 K

4
Atomic nuclei

• The nucleus is itself made up of
• Protons, p (positively charged)
• Neutrons, n (neutral no charge)
• Collectively, these particles are known as
  baryons
• p is slightly less massive than n (0.1
  difference)
• Protons and neutrons bound together by the strong
  nuclear force (exchange of gluons)
• Binding Energy per nucleon few Mev1010 K

5
Elements isotopes

• Number of protons determines element
• Hydrogen 1 proton
• Helium 2 protons
• Lithium 3 protons
• Beryllium 4 protons
• Boron 5 protons
• Carbon 6 proton
• Number of neutrons determines the isotope
• e.g., for hydrogen (1 proton), there are
• three isotopes
• Normal Hydrogen (H or p) no neutrons
• Deuterium (d) 1 neutron
• Tritium (t) 2 neutrons

6
Quarks

• Theres one more level below this, consisting of
  quarks
• Protons Neutrons are made up of trios of quarks
• Up quarks Down quarks
• Proton 2 up quarks 1 down quark
• Neutron 1 up quark 2 down quarks
• There are other kinds of quarks (strange, charm,
  top, bottom quarks) that make up more exotic
  types of particles

7
Quarks

• In particle physics, quarks are one of the two
  basic constituents of matter (the other Standard
  Model fermions are the leptons).
• Antiparticles of quarks are called antiquarks.
  Quarks are the only fundamental particles that
  interact through all four of the fundamental
  forces. The word was borrowed by Murray Gell-Mann
  from the book Finnegans Wake by James Joyce,
  where seabirds give "three quarks", akin to three
  cheers (probably onomatopoetically imitating a
  seabird call, like "quack" for ducks).
• The names of quark flavours (up, down, strange,
  charm, bottom, and top) were also chosen
  arbitrarily based on the need to name them
  something that could be easily remembered and
  used.
• An important property of quarks is called
  confinement, which states that individual quarks
  are not seen because they are always confined
  inside subatomic particles called hadrons (e.g.,
  protons and neutrons) an exception is the top
  quark, which decays so quickly that it does not
  hadronize, and can therefore be observed more
  directly via its decay products. Confinement
  began as an experimental observation, and is
  expected to follow from the modern theory of
  strong interactions, called quantum
  chromodynamics (QCD). Although there is no
  mathematical derivation of confinement in QCD, it
  is easy to show using lattice gauge theory.

8
Nuclear fusion

• Heavier nuclei can be built up from lighter
  nuclei (or free n, p) by fusion
• Need conditions of very high temperature and
  density to overcome repulsion of protons
• These conditions are present only in cores of
  stars and in the early Universe!
• The original motivation of Gamow, Alpher,
  Herman in advocating big bang was that it could
  provide conditions conducive to nuclear reactions

9
The early universe must have been extremely hot
and dense
3 minutes T about 109 K
10
History of Universe according to BIG BANG Theory
11
Planck Era Before Planck time (10-43 sec) No
theory of quantum gravity
12
Photons converted into particle-antiparticle
pairs and vice-versa E mc2 Early
universe was full of particles and radiation
because of its high temperature
13
GUT Era Lasts from Planck time (10-43 sec) to
end of GUT force (10-38 sec)
14
Electroweak Era Lasts from end of GUT force
(10-38 sec) to end of electroweak force (10-10
sec)
15
Particle Era Amounts of matter and antimatter
nearly equal (Roughly 1 extra proton for every
109 proton-antiproton pairs!)
16
Era of Nucleo-synthesis Begins when matter
annihilates remaining antimatter at 0.001
sec Nuclei begin to fuse
17
Era of Nuclei Helium nuclei form at age 3
minutes Universe has become too cool to blast
helium apart
18
Era of Atoms Atoms form at age 380,000
years Background radiation released
19
Era of Galaxies Galaxies form at age 1 billion
years
20
Primary Evidence

• We have detected the leftover radiation from the
  Big Bang.
• The Big Bang theory correctly predicts the
  abundance of helium and other light elements.

21
What have we learned?

• What were conditions like in the early universe?
• The early universe was so hot and so dense that
  radiation was constantly producing
  particle-antiparticle pairs and vice versa
• What is the history of the universe according to
  the Big Bang theory?
• As the universe cooled, particle production
  stopped, leaving matter instead of antimatter
• Fusion turned remaining neutrons into helium
• Radiation traveled freely after formation of atoms

22
Evidence for the Big Bang

• How do we observe the radiation left over from
  the Big Bang?
• How do the abundances of elements support the Big
  Bang theory?

23
How do we observe the radiation left over from
the Big Bang?
24
The cosmic microwave background the radiation
left over from the Big Bang was detected by
Penzias Wilson in 1965
25
Background radiation from Big Bang has been
freely streaming across universe since atoms
formed at temperature 3,000 K visible/IR
26
Background has perfect thermal radiation spectrum
at temperature 2.73 K
Expansion of universe has redshifted thermal
radiation from that time to 1000 times longer
wavelength microwaves
27
WMAP gives us detailed baby pictures of structure
in the universe
28
In early Universe

• At t1s, neutrinos began free-streaming
• At t14s, e? stopped being created and destroyed
• Temperature continued to drop until protons and
  neutrons, if they combined, were not necessarily
  broken apart

29
NUCLEOSYNTHESIS IN THE EARLY UNIVERSE

• Nucleosynthesis the production of different
  elements via nuclear reactions
• Consider universe at t180s
• i.e. 3 minutes after big bang
• Universe has cooled down to 1 billion (109) K
• Filled with
• Photons (i.e. parcels of electromagnetic
  radiation)
• Protons (p)
• Neutrons (n)
• Electrons (e)
• also Neutrinos, but these were freely streaming

30
The first three minutes

• Protons and Neutrons can fuse together to form
  deuterium (d)
• But, deuterium is quite fragile
• Before t180s, Universe is hotter than 1 billion
  degrees.
• High-T means that photons carry a lot of energy
• Deuterium is destroyed by energetic photons as
  soon as it forms

31
After the first 3 minutes

• But, after t180s, Universe has cooled to the
  point where deuterium can survive
• Deuterium formation is the first step in a whole
  sequence of nuclear reactions
• e.g. Helium-4 (4He) formation

32
Protons and neutrons combined to make
long-lasting helium nuclei when universe was 3
minutes old
33
Big Bang theory prediction 75 H, 25 He (by
mass) Matches observations of nearly primordial
gases
34

• Further reactions can give Lithium (Li)
• Reactions cannot easily proceed beyond Lithium
  due to the stability gap more about that later

35

• If this were all there was to it, then the final
  mixture of hydrogen helium would be determined
  by initial number of p and n.
• If equal number of p and n, everything would
  basically turn to 4He Pairs of protons and pairs
  of neutrons would team up into stable Helium
  nuclei.
• Would have small traces of other species
• But we know that most of the universe is
  hydrogen why are there fewer n than p? What
  else is going on?

36
Neutron decay

• Free neutrons (i.e., neutrons that are not bound
  to anything else) are unstable!
• Neutrons spontaneously and randomly decay into
  protons, emitting electron and neutrino
• Half life for this occurrence is 15 mins (i.e.,
  take a bunch of free neutrons half of them will
  have decayed after 15 mins).

37

• While the nuclear reactions are proceeding,
  supply of free neutrons is decaying away.
• So, speed at which nuclear reactions occur is
  crucial to final mix of elements
• What factors determine the speed of nuclear
  reactions?
• Density (affects chance of p/n hitting each
  other)
• Temperature (affects how hard they hit)
• Expansion rate of early universe (affects how
  quickly everything is cooling off and spreading
  apart).

38

• Full calculations are complex. Need to
• Work through all relevant nuclear reactions
• Take account of decreasing density and decreasing
  temperature as Universe expands
• Take account of neutron decay
• Feed this into a computer
• Turns out that relative elemental abundances
  depend upon the quantity ?BH2
• Here, ?B is the density of the baryons
  (everything made of protonsneutrons) relative to
  the critical density.

39

• Full calculations are complex. Need to
• Work through all relevant nuclear reactions
• Take account of decreasing density and decreasing
  temperature as Universe expands
• Take account of neutron decay
• Feed this into a computer
• Turns out that relative elemental abundances
  depend upon the quantity ?BH2
• Here, ?B is the density of the baryons
  (everything made of protonsneutrons) relative to
  the critical density.

40

• We can use the spectra of stars and nebulae to
  measure abundances of elements
• These need to be corrected for reactions in stars
• By measuring the abundance of H, D, 3He, 4He, and
  7Li, we can
• Test the consistency of the big bang model -- are
  relative abundances all consistent?
• Use the results to measure the quantity ?Bh2

41
How do the abundances of elements support the Big
Bang theory?
42
From M.Whites webpage, UC Berkeley
Dependence of abundances on ?BH2
?Bh2
43
Results

• All things considered, we have ?Bh2?0.019.
• If H072km/s/Mpc,
• h0.72
• ?B?0.04
• This is far below ?1!
• Baryons alone would give open universe

?Bh2
44
What have we learned?

• How do we observe the radiation left over from
  the Big Bang?
• Radiation left over from the Big Bang is now in
  the form of microwavesthe cosmic microwave
  backgroundwhich we can observe with a radio
  telescope.
• How do the abundances of elements support the Big
  Bang theory?
• Observations of helium and other light elements
  agree with the predictions for fusion in the Big
  Bang theory

45
(No Transcript)
46
RECAP

• The density parameter for matter is defined as
• Value of ?M very important for determining the
  geometry and dynamics (and ultimate fate) of the
  Universe
• Constraints from nucleosynthesis
• To get observed mixture of light elements, we
  need the baryon density parameter to be ?B?0.037
• If there were only baryonic matter (normal
  stuff made of protons, neutrons, electrons) in
  the Universe, then this would imply that
  ?M?0.037.
• In that case, and if ? were 0, the Universe
  would be open (hyperbolic) and would expand
  forever

47
Standard model evolution diagrams
48
Preview

• But life is more complicated than that
• Much evidence shows that ?M may be 5 or 10 times
  larger than ?B , yet still ?M lt1
• Additional evidence suggests that nevertheless,
  the Universe is flat, with k0 so ?k 0 (i.e.
  neither hyperbolic nor spherical geometrically)
• This implies the cosmological constant ? must be
  nonzeroand in fact, there is observational
  evidence for accelerating expansion!
• Well start with the accounting of all forms of
  mass in the Universe

49
THE MASS OF STARS IN THE UNIVERSE

• Stars are the easiest things to see and study in
  our Universe
• Can study nearby stars in detail
• Can see the light from stars using normal
  optical telescopes in even distant galaxies.
• Butwhat we see is the light, and what were
  interested in is the mass
• Need to convert between the two using the
  mass-to-light ratio M/L.

50
The Sun

• Msun2?1030 kg
• Lsun4?1026 W
• Actual numbers not very instructive
• From now on, we will reference mass-to-light
  ratios to the Sun (Msun/Lsun).

51
Other stars

• Different types of stars have different
  mass-to-light ratios
• Massive stars have small M/L (they shine brightly
  compared with their mass).
• Low-mass stars have large M/L (they are very dim
  compared with their mass).
• Were interested in an average M/L
• Averaging regular stars near to the Sun, we get
  M/L?3 Msun/Lsun

52

• But, we also need to include effect of dead
  stellar remnants
• white dwarfs, neutron stars, black holes.
• and also sub-stellar mass objects
• Called brown dwarfs
• Interior gravity is too low to compress gas and
  initiate fusion at very low luminosity
• All of these have mass M, but very little light
  L.
• They add to the numerator of the average M/L,
  but not to the denominator
• Including the remnants and (smaller) brown dwarf
  contribution, this would increase the
  mass-to-light ratio for spiral galaxies to about
  
• M/L?10 Msun/Lsun

53

• So, can add up the visible star light that we see
  in the Universe, and convert to a mass in stars
  (luminous and non-luminous).
• We get ?L?0.005-0.01
• Comparing with ?B0.037 from nucleosynthesis, we
  see that most baryons cannot be in stars

54
Wheres the rest of the baryonic matter if its
not in stars?

• Galaxy clusters contain a lot of hot gas outside
  of individual galaxies
• Gas temperature of 10-100 million K.
• Can see it using X-ray telescopes.
• Such gas contains a lot of the baryons
• The rest is believed to be in warm/hot (1
  million K) gas in intergalactic space.

X-ray emission from the hot gas trapped in the
Cygnus-A cluster
55
(No Transcript)
56
(No Transcript)
57
Real measurements
In outer parts of galaxies, V and R are based on
measurements of hydrogen gas atoms orbiting
galaxy, rather than stars
58
(No Transcript)
59
Called a dark matter halo
60

• Orbital velocity stays almost constant as far out
  as we can track it
• Means that enclosed mass increases linearly with
  distance
• Mass continues to increase, even beyond the
  radius where the starlight stops
• While there is enough diffuse gas out there to
  track V, it adds only a tiny amount of mass
• So, in these outer regions of galaxies, the mass
  isnt luminous
• This is DARK MATTER.

61
(No Transcript)
62
(No Transcript)
63
(No Transcript)
64

• How big are galaxy halos?
• We dont know!
• But they might be huge maybe 10 times bigger
  than luminous part of the galaxy!
• Add up all the galaxy halos how much mass would
  there be?
• Uncertain - we dont know how far out galaxy
  halos go.
• Somewhere in range ?halos0.1-0.3

65
Non-baryonic dark matter

• This is our first evidence for non-baryonic dark
  matter
• ?B0.04 (nucleosynthesis)
• ?halos0.1-0.3 (galaxy rotation curves)
• So, there is substantially more mass in the
  galaxy halos than could possibly be due to
  baryons!
• Suggests a non-baryonic form of matter may exist
  something not based on protons and neutrons.

66
MASS OF GALAXY CLUSTERS

• Galaxy clusters
• Large groups of galaxies
• Bound together by mutual gravitational attraction
• Lets use same arguments for velocities and radii
  of galaxies in cluster as for V and R of stars in
  galaxies (i.e., based on Newtons laws) to
  measure mass

67
The Virgo cluster
68
Dark matter in clusters

• Find that here is a giant halo of dark matter
  enveloping the galaxy cluster
• Includes the individual halos attached to each
  galaxy in cluster
• Also includes dark matter ripped from individual
  galaxies halos, or never attached to them
• Add up the mass in these cluster halos
• ?cluster0.3
• Some of this mass is in hot gas in the cluster
  (contributing to ?B0.04 from nucleosynthesis),
  but most is non-baryonic dark matter

69
Gravitational lensing

• In some cases, can also measure cluster mass
  using gravitational lensing.
• Get good agreement with dynamical measurements

70
NON-BARYONIC DARK MATTER SUMMARY

• Recap again
• Nucleosynthesis arguments constrain the density
  of baryons (?B?0.037)
• But there seems to be much more mass in galaxy
  and cluster halos (total ?Matter0.3)
• So, most of the matter in the Universe is not
  baryonic!
• what is it????

71
(No Transcript)
72
The cosmic concordance

• What is our universe like?
• Matter content?
• Geometry (flat, spherical, hyperbolic)?
• Anything else strange?
• Remarkable agreement between different
  experimental techniques
• Cosmic concordance parameters

73
Measurements of the matter content of the
Universe (recap)

• Primordial nucleosynthesis
• Theory predicts how present light element
  abundances (4He, 3He, D, 7Li) depend on mean
  baryon density
• Observed abundances ? ?B?0.04
• Galaxy/galaxy-cluster dynamics
• Look at motions of stars in galaxies, or galaxies
  in galaxy clusters
• Infer presence of large quantities of dark
  matter which gravitationally affects observed
  objects but cannot be seen with any telescope

74

• Analysis of galaxy motions suggests a total
  matter density of ?Matter?0.3
• Same conclusion from gravitational lensing by
  clusters (light from background objects is bent
  due to GR effects)

75

• First stunning conclusion
• Compare ?B?0.04 and ?Matter?0.3
• Normal matter only accounts for about 1/8 of the
  total matter thats out there!
• Dark matter provides ?DM?0.26
• Were made of the minority stuff!

76

• Can be confirmed by taking an inventory of a
  cluster, where diffuse gas is hot and emits
  X-rays
• Find that about 1/8 of a clusters mass is in
  baryons
• We believe that clusters should be representative
  samples of the universe
• Confirms ?DM?0.26

77
MEASURING THE GEOMETRY OF THE UNIVERSE

• Recall that universe with different curvature has
  different geometric properties
• Adding up the angles in a triangle,
• Flat universe(k0) angles sum to 180?
• Spherical universe (k1) angles sum to gt180?
• Hyperbolic universe (k-1) angles sum to lt180?
• Similarly, for a known length L at a given
  distance D, the angular size on the sky varies
  depending on the curvature of space
• Flat universe (k0) angular size ?L/D
• Spherical universe (k1) angular size ?gtL/D
• Hyperbolic universe (k-1) angular size ?ltL/D

78
L
L
L
D
k-1
k0
k1
79
Angular size of fluctuations in the CBR

• Remember the cosmic microwave background
• It has fluctuations, with average separations
  corresponding to a known scale L at the distance
  where light last interacted with matter
  (matter/radiation decoupling)
• Distance D to this surface of last scattering
  is also known
• Can use apparent angular separations of
  fluctuations compared to L/D to infer geometry of
  Universe

80
us
L
D
81
Flat universe!

• Result
• The universe is flat
• In terms of omega curvature parameter,
• ?k0, i.e k0
• Recall that the sum of all three omega parameters
  as measured at present time must be 1
• How do we reconcile ?k0 with our measurement of
  the matter density, which indicates ?M0.3?
• There must be a nonzero cosmological constant,
  ??0.7!